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Information on the composition of the Earth as a whole has been deduced from: (a) cosmochemical models, which assume that the compositions of all members of the solar system are related, and that the bulk composition of the Earth can be inferred from the abundances of non-volatile elements in the Sun and some primitive meteorites (see chondritic earth model); and (b) geophysical evidence, e.g. seismic data, density determinations, and magnetic surveys. The three layers, core, mantle, and crust, of which the Earth is formed differ markedly in their composition. The core and mantle make up more than 99% of the Earth's mass but their compositions can be only inferred, unlike that of the crust (see crustal abundance of elements). The density and magnetic field of the Earth, information from seismic surveys, and the existence of iron–nickel meteorites lead to the conclusion that the core is predominantly iron, with a small proportion of lower-density element(s). Nickel is largely excluded because it would make the core too dense. The nature of the light component is controversial, but may be sulphur, carbon, oxygen, silicon, and potassium. From seismic evidence the mantle appears to be composed of dunite, peridotite, and eclogite, rocks similar in composition to chondritic meteorites. The upper mantle is probably formed from dense silicates of iron and magnesium, with silicon and magnesium oxides becoming commoner with depth. Information regarding the bulk composition of the Earth is fundamental to resolving such questions as the relationship between the Earth and the Moon. Perhaps the most critical single feature of the Earth's bulk composition is its content of K, U, and Th, since the radioactive isotopes of these elements control radioactive heat production, and therefore the thermal and geologic history of the Earth. See also cosmic abundance of elements; meteoritic abundance of elements; and solar abundance of elements.

(a) cosmochemical models, which assume that the compositions of all members of the solar system are related, and that the bulk composition of the Earth can be inferred from the abundances of non-volatile elements in the Sun and some primitive meteorites (see chondritic earth model); and (b) geophysical evidence, e.g. seismic data, density determinations, and magnetic surveys.